The present invention relates to a feed for farmed fish. The invention further comprises a method to provide fish of high quality and use of additional components to prepare said feed.
The main purpose of the invention is to contribute to low cost production of high quality salmonides like rainbow trout and Atlantic salmon.
For the aquaculture industry it has long been an economic problem that farmed fish like salmon and trout do not naturally achieve the same strongly red colour as the wild species. Such farmed fishes are palely red, unless large amount of synthetic red pigments are supplemented to the feed, and therefore not as attractive as the wild fish to the customers.
In a world wide investigation, it is shown that next to product freshness the colour of the flesh is the most important quality criteria for the Atlantic salmon and thus fundamental for market acceptance and price (Koteng, 1992; Markedsunders.o slashed.kelse Norsk Laks. FNL. Bergen. Norway). Hence, to obtain an image and high market value of salmonides with regard to characteristic red or pink flesh pigmentation, pigments like astaxanthin are added to the feeds to make the fish more red.
The deposition rate of astaxanthin in the muscle of salmonides depends on several factors. These include the dietary source of astaxanthin, the concentration of astaxanthin, dietary fat content, and physiological factors such as fish size, growth rate, maturation, and genetic factors (Torrissen et al., 1995; &lt;&lt;Astaxanthin deposition in the flesh of Atlantic salmon; Salmo salar L., in relation to dietary astaxanthin concentration and feeding period&gt;&gt;, in Aquaculture nutrition, 77-84).
The supplement of pigments in the diets of salmonides represents a considerable part of the feed cost. In Norway for example, the astaxanthin supplement in Atlantic salmon diets represents about 20% of the feed costs, or close to 10% of the total production costs. This is mainly due to the low retention of dietary astaxanthin. Thus, increasing the dietary level of astaxanthin from 60 to 100 mg/kg gives an insignificant increase in muscle concentration of only 3% (Torrissen et al., 1995), and several studies have shown that increasing the dietary concentration of astaxanthin above 40 mg/kg gives a significantly reduced relative astaxanthin retention (%). Hence, to improve quality and to reduce production costs, it would be of great importance to develop a feed which improve the retention of dietary astaxanthin.
Another problem in fish farming is development of ulcers on the fish skin, especially for big salmon in the winter when the temperature is low, and for 0.sup.+ -smolt the first winter at sea. Ulcers increase mortality and reduce the price since slaughtered fish with ulcers are down classified from superior to production quality.
M. Staumes et al. (Aquaculture, vol. 90, 1990; &lt;&lt;Distanced, water-filled stomach in sea farmed rainbow trout)&gt;&gt;., pp. 333-343) have reported that a large amount of rainbow trout which are cultured in salt water develop a considerably enlarged abdomen. This is connected to a great enlarged stomach containing water and fat. There is today no treatments for fish suffering from this disease. In addition to the above mentioned problems, the rainbow trout, and in some cases also the salmon, have problems with the fat digestibility. The condition characterised as &lt;&lt;fat belching&gt;&gt; is put in association with the occurrence of water-filled stomach (O. Einen, Hovedoppgave ved NLH, 1989: &lt;&lt;Eit studium av vassbuk hos regnbogeaure&gt;&gt;).
The main object of the present invention is therefore to provide a fish feed which improves the quality of the fish with regard to colour, ulcers and fat digestibility.
The inventors wanted to examine the effect of urea and/or trimethylamine oxide (TMAO) in the feed with regard to promotion of growth and saltwater tolerance of the fish. Commercial fish feed supplemented with trimethylamine oxide (TMAO) is known from for example our own Norwegian patent no. 920083 where TMAO is added to the feed to achieve an optimum level of bio-available dietary iron, and French patent no. 2561871 where TMAO was supplemented to make the feed more appetising to the fish. From U.S. Pat. No. 3,930,018 using isobutylidene diurea as an additional component in the feed for pisciculture for the purpose of promoting growth is known. Furthermore, the utilization of dietary urea in rainbow trout has been studied by Kaushik. et al. in 1983 (&lt;&lt;Utilization of dietary urea in rainbow trout&gt;&gt;, Ann. Nutr. Metab., 27: 94-106).